The UMTS standard specifies a particular power loop control mechanism for adjusting the power transmitted both by the base station to the mobile user (User Equipment UE)—in the case of the downlink—as well as the power transmitted by the latter to the Base station in the case of the uplink. See in particular 3GPP specification 25.211, 25.212, 25.213, 25.214.
The power control is divided into two parts, fast power control also called inner loop control and outer loop control. The fast power control is used to counteract the effects of fast fading by adjusting the transmitting power of the mobiles in order to achieve a given Signal to Interference Ratio (SIR) target.
Outer loop power control is not specified in 3GPP and is used to maintain a certain quality in terms of Block Error Rate (BLER). This is done by comparing a measure BLER value with a BLERtarget and using the difference to regulate the SIRtarget used by the fast power control.
Those two mechanisms, the former Outer Loop Power Control and a latter Inner Loop Power control mechanism, are illustrated in FIG. 1 showing UE 1 and base station 2 regulating the power of the communication by means of Transmit Power Control (TPC) commands received in the uplink and the downlink for keeping the received uplink Signal-to-Interference Ratio (SIR) at a given SIR target. The base station transmitter is capable of changing the output power in the slot immediately after the TPC_cmd can be derived.
With respect to FIG. 1, one should take note of the fact that SIRest is equal to the averaged SIR estimation and SIR target is equal to the value of SIR which is needed to get the required BLER target. DeltaTPC adjust is the power offset in dB that can be applied from one slot to the next to the dedicated channel.
The outer loop adjustment—which is the object of the present invention—is an internal process performed by the UE adapting the SIR Target in order to reach the quality of service set by the network with the aim of maintaining constant the level of the BLER even with a changing environment. To achieve this, the UE estimates the BLER from the number of CRC (Code Redundancy Check) attached to the transmitted data. Checking the CRC the UE can detect if a transmitted block is corrupted or not.
Typical values for the BLER Target are 0.1, 0.01, 0.001, for connection with high quality of service a BLER target of 10−4, would be required.
More information relating to this procedure can be found in the following reference documents:    “On setting reverse link target SIR in a CDMA system”, A. Sampath, P. S. Kumar, J. M. Holzman in proc. of IEEE 47th Vehicular Technology Conference, 1997.
Outer loop power control techniques which are known in the art show a significant drawback resulting in the great number of blocks or frames which need to be processed before issuing any feedback value regarding the actual SIR characterizing the wireless communication. For instance, a BLER target equal to 10−3 requires in practice several thousands of blocks to achieve minimum accuracy.
Clearly, such known outer loop power control techniques are inappropriate for a number of situations where the environment is rapidly changing, including the following cases:
the moving of the User Equipment and, correspondingly, an environment switching from a static environment to a Fading environment;
an ideal Line Of Sight communication might suddenly disappear and be replaced by a strongly attenuated multipath wireless communication;
the UE receives multi-TrChs (Transport Channels), for instance 1 Turbo and 1 Viterbi, and the base station decides to switch from the turbo decoding to a viterbi decoding thus resulting in a decrease in the efficiency of the error correcting process. In practice, one notes that a SIR Target for Turbo can be 2 dBs smaller than SIR Target Viterbi for same BLER Target.
UE receives multi-TrChs with different BLER target, the TrCh with the bigger BLER target is continuously received while the other is not. As soon as the TrCh with smaller BLER target is transmitted again the performance of such are severely degraded.
In these circumstances, the quality of service is exposed to sudden deterioration, requiring a prompt increase of the SIR target by the outer loop control so as to avoid serious communication problems or, even, the loss of the connection (Call drop).
Known outer loop power control techniques, because they exclusively are based on the BLER, are incapable of providing fast adaptation to such changing environment.
Thus, there is therefore a desire to improve the situation.